Vomiting during pregnancy has been described since 2,000 B.C., but few studies have attempted to describe its epidemiology. First trimester registrants in the Collaborative Perinatal Project were screened for the presence of vomiting. Vomiting was more common in blacks, primigravidae, young women, heavy women, non-smokers and women with less education. The absence of vomiting placed a woman at increased risk of fetal loss. There was a modest protective effect on preterm delivery, and no effect on the incidence of low birth weight.